PURPOSE: An ink composition in which a leuco dye and a color developer are dissolved in a solvent and the solution is dried, when needed, to develop the color, thus needing no physical separation of the leuco dye from the developer.

CONSTITUTION: A colorless or slightly coloring leuco dye such as triphenylmethane phthalide and a color developer such as an organic acid, e.g., oxalic acid, or a phenol, e.g., catechol are added to a solvent that does not develop the color, even when both of them are dissolved at the same time, such as alkylnaphthalene, when needed, further additives are admixed to give the objective ink composition.

USE: Dye, paint, ink, heat-sensitive or pressure-sensitive recording material or toner for electronic photography.